The past 20 years has seen the increasing utilization of supercritical fluids in a host of chemical processes, particularly in supercritical fluid extraction. Typically, in supercritical fluid extraction, a dense gas, which serves as solvent or extractant, is brought into contact with the solid or liquid phase of another material (solute) at high pressure and moderate temperature. Slight changes in the temperature or pressure of the solvent gas generally cause large changes in the density of the solvent which can affect the ability of the gas to dissolve the solute. Changes in the temperature or pressure of the extractant can result in the complete precipitation of the solute.
Examples of applications of supercritical fluid extraction include decaffeinating coffee (U.S. Pat. Nos. 3,843,824; 4,246,291; 4,247,570; 4,251,559; 4,255,458; 4,260,639; and 4,276,315), extracting oil from seeds and foods, such as potato chips, fractionating low vapor-pressure oils and fluids, extracting toxic organics from soils, and stripping organics from metal and mineral surfaces.
A pure material becomes a supercritical fluid when its temperature and pressure both equal or exceed the critical temperature, T.sub.c, and the critical pressure, P.sub.c, of the material. The critical point of a substance is characterized by the temperature T.sub.c and pressure P.sub.c and is a well-defined thermodynamic property where the pure vapor phase of a material has identical properties with a pure liquid phase of the material at the same pressure and temperature, i.e. the gaseous and liquid phases are identical. For example, carbon dioxide has a critical temperature of 87.8.degree. F. (31.degree. C.) and a critical pressure of 1066.3 psia; water has a critical pressure of 3206.2 psia and a critical temperature of 705.4.degree. F. (374.1.degree. C.); and nitrogen has a critical temperature of 227.2.degree. K. and a critical pressure of 492.3 psia. The surface meniscus normally separating liquid and vapor phases vanishes at the critical point; therefore, there is no distinction between liquid and vapor phases at supercritical pressures and temperatures.
Generally, the viscosities of supercritical fluids are intermediate to typical liquid and gas viscosities. Another interesting and useful property of supercritical fluids is that they have solvation power similar to liquids which is directly related to critical fluid density.
Supercritical fluids are typically circulated in systems, such as those mentioned above, by compressors. However, the high pressures at which supercritical fluid systems operate place great demands on compressor components, particularly on the seals, and such seals require frequent replacement. Repair and/or compressor maintenance of supercritical fluid compressors requires depressurization of part of the supercritical fluid system and repressurization before the system can be placed back in service. Furthermore, to eliminate potential contaminants in such systems, the depressurized section is commonly evacuated prior to repressurization. All of the steps associated with the repair and maintenance of supercritical compressors is expensive in terms of labor costs and system down-time, as well as time-consuming. Therefore, it may be appreciated that there is a need for a system or method that is capable of circulating a dense gaseous fluid without the need for a compressor.
Supercritical fluids have also been employed as heat transfer media in heat pipes. A heat pipe is a device that provides a substantial amount of heat transfer through a given surface area. E. Schmidt, a German engineer, demonstrated a heat pipe which provided a heat flux more than 4,000 times greater than the heat transfer rate of a comparably sized copper rod operating between the same temperature limits.
A heat pipe basically includes a condensible gas or vapor and a liquid within a pipe that is sealed at both ends. When the evaporator end of the pipe is subjected to a source of heat energy, the liquid stored in the evaporator end is vaporized, and the vapor moves to the condenser end of the pipe. If the condenser end of the pipe is in thermal contact with a cool environment that absorbs heat energy from the vapor, the vapor condenses and either falls back or is pulled back by capillary action through a wick to the evaporator end. Heat transfer continues as long as the ends of the pipe are subjected to temperatures sufficient to vaporize the fluid at the evaporator end and condense it at the condensing end. Even small temperature differences can be enough to transfer significant amounts of heat energy from end to end of the heat pipe.
However, heat pipes have certain performance limitations which are disadvantageous. Frictional losses attributable to the viscosity of liquid flow through the pipe or the wick restrict the overall practical length of a heat pipe, and heat pipes are generally not suitable for transferring heat energy beyond a distance of about 10 meters. A wick in the evaporator end can be dried out by a high heat flux causing the heat pipe to no longer function. Also, the pipe size should be large enough to prevent capillary forces from interfering with the gas-liquid transport, and as a result, application to small components such as electronics is limited.
Thus, it may be appreciated that there is a need for a heat transfer device which is capable of providing high heat flux, but which is not subject to the performance limitations of a heat pipe. Also, there is a need for a heat transfer system and method that may be employed to transfer heat energy over longer distances than are practical with a heat pipe and/or which can be incorporated into oxidation or extraction processes.